JavaScript 全局变量未更改

Question

我是 JavaScript 的新手，我制作了一个简单的秒表构造函数：

function Stopwatch() {
    var started = false;
    var elapsed;
    var start;
    d = new Date();
    this.duration = function() {
        if (started) {
            throw "Stopwatch is still running"
        } else {
            return (elapsed);
        }
    }
    this.start = function() {
        if (started) {
            throw "Stopwatch has already started"; 
        } else {
            start = d.getTime();
            started = true;
        }
    }
    this.stop = function() {
        if (!started) {
            throw "Stopwatch hasn't started";
        } else {
            elapsed = d.getTime() - start;
            started = false;
        }
    }
    this.reset = function() {
        started = false;
        elapsed = 0;
    }
}
let sw = new Stopwatch();
sw.start();

setTimeout(function () {
    sw.stop();
    console.log(sw.duration());
}, 500);

错误的逻辑工作正常，但每当我调用 sw.duration() 时，它总是 returns 零，我不确定为什么会这样，因为我使用的是 var让。请告诉我。

Answer 1

您只调用了一次 new Date，在构造函数的开头。使用 Date.now 代替（获取数字，避免必须调用 .getTime），并且不仅在需要分配给 start 时调用它，而且在 .stop 中调用它, 得到 elapsed.

function Stopwatch() {
    var started = false;
    var elapsed;
    var start;
    this.duration = function() {
        if (started) {
            throw "Stopwatch is still running"
        } else {
            return (elapsed);
        }
    }
    this.start = function() {
        if (started) {
            throw "Stopwatch has already started"; 
        } else {
            start = Date.now();
            started = true;
        }
    }
    this.stop = function() {
        if (!started) {
            throw "Stopwatch hasn't started";
        } else {
            elapsed = Date.now() - start;
            started = false;
        }
    }
    this.reset = function() {
        started = false;
        elapsed = 0;
    }
}
let sw = new Stopwatch();
sw.start();
setTimeout(() => {
  sw.stop();
  console.log(sw.duration());
}, 500);

JavaScript 全局变量未更改

JavaScript Global Variable Not Changing

javascript

var

global

let